Lithographic apparatus and device manufacturing method

ABSTRACT

In a lithographic projection apparatus, a liquid supply system maintains liquid in a space between the projection system and the substrate. The liquid supply system may further include a de-mineralizing unit, a distillation unit, a de-hydrocarbonating unit, a UV radiation source, and/or a filter configured to purify the liquid. A gas content reduction device may be provided to reduce a gas content of the liquid. A chemical may be added to the liquid using an adding device to inhibit lifeform growth and components of the liquid supply system may be made of a material which is non-transparent to visible light such that growth of lifeforms may be reduced.

The present application is a continuation of co-pending U.S. patentapplication Ser. No. 13/240,867, filed Sep. 22, 2011, which is acontinuation of co-pending U.S. patent application Ser. No. 12/766,609,filed Apr. 23, 2010, which is a continuation of U.S. patent applicationSer. No. 10/924,192, filed Aug. 24, 2004, now U.S. Pat. No. 7,733,459,which claims priority from European patent application EP 03255376.0,filed Aug. 29, 2003, the entire contents of each of the foregoingapplications is hereby incorporated by reference.

FIELD

The present invention relates to a lithographic projection apparatus anda device manufacturing method.

BACKGROUND

The term “patterning device” as here employed should be broadlyinterpreted as referring to any device that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such patterning devices include:

—A mask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

—A programmable mirror array. One example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, theundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. An alternative embodiment of a programmable mirror arrayemploys a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuator. Once again, themirrors are matrix-addressable, such that addressed mirrors will reflectan incoming radiation beam in a different direction to unaddressedmirrors; in this manner, the reflected beam is patterned according tothe addressing pattern of the matrix-addressable mirrors. The requiredmatrix addressing can be performed using suitable electronic means. Inboth of the situations described hereabove, the patterning device cancomprise one or more programmable mirror arrays. More information onmirror arrays as here referred to can be gleaned, for example, from U.S.Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patentapplications WO 98/38597 and WO 98/33096, which are incorporated hereinby reference. In the case of a programmable mirror array, the supportstructure may be embodied as a frame or table, for example, which may befixed or movable as required.

—A programmable LCD array. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference. Asabove, the support structure in this case may be embodied as a frame ortable, for example, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning devices ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at one time; such an apparatus is commonly referredto as a stepper. In an alternative apparatus—commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “projection lens”; however, this term should bebroadly interpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCTpatent application WO 98/40791, incorporated herein by reference.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.)

One proposal is to submerse the substrate or both substrate andsubstrate table in a bath of liquid (see, for example, U.S. Pat. No.4,509,852, hereby incorporated in its entirety by reference).

Another of the solutions proposed is for a liquid supply system toprovide liquid on only a localized area of the substrate and in betweenthe final element of the projection system and the substrate (thesubstrate generally has a larger surface area than the final element ofthe projection system). One way which has been proposed to arrange forthis is disclosed in WO 99/49504, hereby incorporated in its entirety byreference. As illustrated in FIGS. 2 and 3, liquid is confined to alocalized area by being supplied by at least one inlet IN onto thesubstrate, preferably along the direction of movement of the substraterelative to the final element, and by being removed by at least oneoutlet OUT after having passed under the projection system. That is, asthe substrate is scanned beneath the element in a −X direction, liquidis supplied at the +X side of the element and taken up at the −X side.FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet IN and is taken up on the other side of the element by outletOUT which is connected to a low pressure source. In the illustration ofFIG. 2 the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of in- and out-letspositioned around the final element are possible, one example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.

Another solution which has been proposed is to contain the liquid to alocalised area of the substrate with a seal member which extends alongat least a part of a boundary of the space between the final element ofthe projection system and the substrate table. The seal member issubstantially stationary relative to the projection system in the XYplane though there may be some relative movement in the Z direction (inthe direction of the optical axis). A seal is formed between the sealmember and the surface of the substrate. In an embodiment, the seal is acontactless seal such as a gas seal.

SUMMARY

The properties of the immersion liquid should be carefully controlledsuch that its optical properties remain constant and so that elements ofthe supply and projection systems are not contaminated with deposits.

Accordingly, it would be advantageous, for example, to provide a liquidsupply system in which the quality of immersion liquid can becontrolled.

According to an aspect of the invention, there is provided alithographic apparatus comprising:

an illumination system configured to condition a radiation beam;

a support constructed to hold a patterning device, the patterning devicebeing capable of imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beamonto a target portion of the substrate; and

a liquid supply system configured to at least partly fill a spacebetween the projection system and the substrate with a liquid, theliquid supply system comprising a liquid purifier configured to purifythe liquid.

In this way the lithographic projection apparatus may, for example, beattached to a normal main water supply rather than requiring a source ofpre-purified water. This is advantageous as purified water can beexpensive and the amount remaining would clearly require monitoring sothat it does not run out. In another embodiment, immersion liquids otherthan water may be used, for example, because water may not be suitablefor use with a projection beam of 157 nm wavelength.

In an embodiment, the liquid purifier comprises a (water) distillationunit, and additionally or alternatively the liquid purifier may comprisea (water) demineralizer. In this way, for example, water supplied from anormal main water supply can be supplied to the lithographic projectionapparatus where the water is brought to a purity acceptable for use asimmersion liquid by choice of purification units. Of course otheradditions may be required to the water such as, for example, wettingagents. If the immersion liquid is not water, other types of purifiermay be needed in addition to or instead of the distillation unit anddemineralizer.

In an embodiment, the (water) demineralizer is a reverse osmosis unit.

In a further embodiment, the liquid purifier comprises a filter, whichmay be dynamically isolated from one or more further components in theliquid supply system. The dynamic isolation of the filter helps toprevent clusters of particles which may form in the filter from breakingup and being emitted downstream. Thus, particle contamination of thesubstrate may be reduced and, in turn, yield may be increased.

In an embodiment, the liquid supply system includes a re-circulationmechanism configured to re-use immersion liquid in the space withoutpurifying the immersion liquid for a re-use. Such a system may beadvantageous, for example, because immersion liquid may be re-usedwithout re-purifying thereby improving the economy of the lithographicprojection apparatus.

In a further embodiment of the invention, there is provided alithographic apparatus comprising:

an illumination system configured to condition a radiation beam;

a support constructed to hold a patterning device, the patterning devicebeing capable of imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beamonto a target portion of the substrate; and

a liquid supply system configured to at least partly fill a spacebetween the projection system and the substrate with a liquid, theliquid supply system comprising an ultra-violet radiation sourceconfigured to irradiate the liquid prior to entry into the space.

An ultra-violet source, which is a source other than the projection beamof the lithographic projection apparatus, may be effective to killlifeforms present in the immersion liquid (water) thereby preventingfurther growth. Such lifeforms may include algae which would otherwisecontaminate the lithographic projection apparatus.

In another embodiment of the invention, there is provided a lithographicprojection apparatus lithographic projection apparatus comprising:

an illumination system configured to condition a radiation beam;

a support constructed to hold a patterning device, the patterning devicebeing capable of imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beamonto a target portion of the substrate; and

a liquid supply system configured to at least partly fill a spacebetween the projection system and the substrate with a liquid, theliquid supply system comprises a component configured to prevent theliquid from being irradiated by visible light.

In an embodiment, the component comprises a container or enclosurenon-transparent to visible light surrounding the liquid supply system.In another embodiment, the component comprises conduits which arenon-transparent to visible light configured to supply the liquid from aliquid source to the space. Lifeforms, such as algae, typically requirevisible light so that they can photosynthesize and grow. By preventingvisible light from reaching the immersion liquid, any lifeforms withinthe immersion liquid which require light for life will die. In this waythe quality of the immersion liquid may be maintained, if not improved,and contamination reduced.

In another embodiment of the invention, there is provided a lithographicprojection apparatus lithographic projection apparatus comprising:

an illumination system configured to condition a radiation beam;

a support constructed to hold a patterning device, the patterning devicebeing capable of imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beamonto a target portion of the substrate; and

a liquid supply system configured to at least partly fill a spacebetween the projection system and the substrate with a liquid, theliquid supply system comprises a device configured to add alifeform-growth inhibiting chemical to the liquid. In this solution,lifeforms such as algae may be killed by chemical attack.

According to another aspect of the present invention, there is provideda liquid for use in a space between a projection system of an immersionlithographic projection apparatus and a substrate to be imaged, theliquid comprising a lifeform-growth inhibiting chemical. Such animmersion liquid may lead to less contamination and may be more easilycontrolled in composition than an immersion liquid which does notinclude a lifeform-growth inhibiting chemical.

According to another aspect, there is provided a lithographic projectionapparatus, comprising:

an illumination system configured to condition a radiation beam;

a support constructed to hold a patterning device, the patterning devicebeing capable of imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beamonto a target portion of the substrate; and

a liquid supply system configured to at least partly fill a spacebetween the projection system and the substrate with a liquid, theliquid being water or an aqueous solution having one or more of thefollowing properties (a) to (f):

(a) an electrical conductivity of from 0.055 microSiemens/cm to 0.5microSiemens/cm;

(b) a pH of from 5 to 8 or from 6 to 8;

(c) a content of organic compounds of 5 ppb or less or of 1 ppb or less;

(d) a particle content of no more than 2 particles having a dimension of50 nm or greater per ml of liquid, or of no more than 0.5 particleshaving a dimension of 50 nm or greater per ml of liquid;

(e) a dissolved oxygen concentration of 15 ppb or less or of 5 ppb orless; and

(f) a silica content of 500 ppt or less or of 100 ppt or less.

In this embodiment, the immersion liquid may have a high purity, leadingto a reduction in contamination of various elements in the system whichcontact the liquid, and helping to avoid optical changes orimperfections. The immersion liquid may be purified using a liquidpurifier which is incorporated into the lithographic apparatus asdescribed above, or using a remote purification system (e.g. apoint-of-use filter or a purification unit that supplies liquid to thelithographic apparatus as well as other liquid users). In particular,the apparatus of this embodiment may avoid or lessen the impact of oneor more of the following difficulties:

-   -   liquid stains on the optical elements and/or on the substrate,        caused by immersion liquid drying on or being evacuated from the        surface of the element/substrate;    -   contamination of outer elements of the projection system by        organic species;    -   printing defects caused by particles or bubbles in or close to        the focus plane;    -   optical defects such as straylight;    -   damage to the resist through reaction with materials in the        immersion liquid (e.g. bases) and contamination of the resist        surface through deposition of impurities.

According to a further aspect of the present invention, there isprovided a liquid for use in a space between a projection system of animmersion lithographic projection apparatus and a substrate to beimaged, wherein the liquid has one or more of the following properties(a) to (f):

(a) an electrical conductivity of from 0.055 microSiemens/cm to 0.5microSiemens/cm;

(b) a pH of from 5 to 8 or from 6 to 8;

(c) a content of organic compounds of 5 ppb or less or of 1 ppb or less;

(d) a particle content of no more than 2 particles having a dimension of50 nm or greater per ml of liquid, or of no more than 0.5 particleshaving a dimension of 50 nm or greater per ml of liquid;

(e) a dissolved oxygen concentration of 15 ppb or less or of 5 ppb orless; and

(f) a silica content of 500 ppt or less or of 100 ppt or less.

Such an immersion liquid may help to avoid contamination and to avoid orreduce the difficulties mentioned in the above paragraph.

-   -   According to a further aspect of the invention there is provided        a device manufacturing method, comprising:    -   providing a liquid to at least partly fill a space between a        projection system of a lithographic apparatus and a substrate;        and    -   projecting a patterned beam of radiation through the liquid onto        a target portion of the substrate; and    -   comprising    -   providing untreated water as the liquid to the lithographic        apparatus and purifying the untreated water using a liquid        purifier immediately prior to providing the liquid to the space;        or    -   irradiating the liquid with ultra-violet radiation prior to        providing the liquid to the space; or    -   providing the liquid from a liquid source to the space via a        conduit which is non-transparent to visible light; or    -   providing a lifeform growth inhibiting chemical to the liquid        prior to providing the liquid to the space; or    -   providing water or an aqueous solution as the liquid, the water        or the aqueous solution having one or more of the following        properties (a) to (f):

(a) an electrical conductivity of from 0.055 microSiemens/cm to 0.5microSiemens/cm;

(b) a pH of from 5 to 8, preferably from 6 to 8;

(c) a content of organic compounds of 5 ppb or less, preferably 1 ppb orless;

(d) a particle content of no more than 2 particles having a dimension of50 nm or greater per ml of immersion liquid, preferably no more than 0.5particles having a dimension of 50 nm or greater per ml of immersionliquid;

(e) a dissolved oxygen concentration of 15 ppb or less, preferably 5 ppbor less; and

(f) a silica content of 500 ppt or less, preferably 100 ppt or less.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 illustrates a liquid supply system according to an embodiment ofthe invention;

FIG. 3 illustrates, in plan, the system of FIG. 3;

FIG. 4 illustrates another liquid supply system according to anembodiment of the invention; and

FIG. 5 illustrates a liquid supply system from a liquid source todisposal according to an embodiment of the present invention.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

-   -   a radiation system Ex, IL, for supplying a projection beam PB of        radiation (e.g. DUV radiation), which in this particular case        also comprises a radiation source LA;    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g. a reticle), and connected to a first        positioning device for accurately positioning the mask with        respect to item PL;    -   a second object table (substrate table) WT provided with a        substrate holder for holding a substrate W (e.g. a resist-coated        silicon wafer), and connected to a second positioning device for        accurately positioning the substrate with respect to item PL;    -   a projection system (“projection lens”) PL (e.g. a refractive        system) for imaging an irradiated portion of the mask MA onto a        target portion C (e.g. comprising one or more dies) of the        substrate W.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning device, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. an excimer laser) produces a beam of radiation. Thisbeam is fed into an illumination system (illuminator) IL, eitherdirectly or after having traversed a conditioner, such as a beamexpander Ex, for example. The illuminator IL may comprise adjustingmeans AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam. In addition, it will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through theprojection system PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the second positioning device (andinterferometer IF), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of the beamPB. Similarly, the first positioning device can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval of the mask MA from a mask library, or during ascan. In general, movement of the object tables MT, WT will be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a stepper (as opposed to astep-and-scan apparatus) the mask table MT may just be connected to ashort stroke actuator, or may be fixed.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at one time (i.e. a single “flash”)onto a target portion C. The substrate table WT is then shifted in the Xand/or Y directions so that a different target portion C can beirradiated by the beam PB;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the Y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the projection system PL(typically, M=1/4 or 1/5). In this manner, a relatively large targetportion C can be exposed, without having to compromise on resolution.

FIG. 4 shows a liquid reservoir 10 between the projection system PL anda substrate W which is positioned on the substrate stage WT. The liquidreservoir 10 is filled with a liquid 11 having a relatively highrefractive index, provided via inlet/outlet ducts 13. The liquid may bewater (as in this description) but can be any suitable liquid. Theliquid has the effect that the radiation of the projection beam is ashorter wavelength in the liquid than in air or in a vacuum, allowingsmaller features to be resolved. It is well known that the resolutionlimit of a projection system is determined, inter alia, by thewavelength of the projection beam and the numerical aperture of thesystem. The presence of the liquid may also be regarded as increasingthe effective numerical aperture. Furthermore, at fixed numericalaperture, the liquid is effective to increase the depth of field.

In an embodiment, the reservoir 10 forms a seal, e.g., a contactlessseal, to the substrate W around the image field of the projection systemPL so that the liquid is confined to fill the space between thesubstrate's primary surface, which faces the projection system PL, andthe final optical element of the projection system PL. The reservoir isformed by a seal member 12 positioned below and surrounding the finalelement of the projection system PL. Thus, the liquid containment systemLCS provides liquid on only a localized area of the substrate. The sealmember 12 forms part of the liquid containment system LCS for fillingthe space between the final element of the projection system and anobject, such as a substrate W or a sensor, on the substrate table WTwith a liquid. This liquid is brought into the space below theprojection system and within the seal member 12. The seal member 12extends a little above the bottom element of the projection system andthe liquid rises above the final element so that a buffer of liquid isprovided. The seal member 12 has an inner periphery that at the upperend closely conforms to the shape of the projection system or the finalelements thereof and may, e.g. be round. At the bottom the innerperiphery forms an aperture which closely conforms to the shape of theimage field, e.g. rectangular, though this is not necessarily so. Theprojection beam passes through this aperture.

The liquid 11 is confined in the reservoir 10 by a seal device 16. Asillustrated in FIG. 4, the seal device is a contactless seal, i.e. a gasseal. The gas seal is formed by gas, e.g. air or synthetic air, providedunder pressure via inlet 15 to the gap between seal member 12 andsubstrate W and extracted by first outlet 14. The over pressure on thegas inlet 15, vacuum level on the first outlet 14 and the geometry ofthe gap are arranged so that there is a high-velocity gas flow inwardstowards the optical axis of the apparatus that confines the liquid 11.As with any seal, some liquid is likely to escape, for example up thefirst outlet 14.

FIGS. 2 and 3 also depict a liquid reservoir defined by inlet(s) IN,outlet(s) OUT, the substrate W and the final element of projectionsystem PL. Like the liquid containment system of FIG. 4, the liquidsupply system illustrated in FIGS. 2 and 3, comprising inlet(s) IN andoutlet(s) OUT, supplies liquid to a space between the final element ofthe projection system and a localized area of the primary surface of thesubstrate.

Both of the liquid supply systems of FIGS. 2 and 3 and of FIG. 4 as wellas other solutions, such as a bath in which the substrate W or the wholesubstrate table WT is immersed, can be used with the liquid supplysystem of an embodiment of the present invention which is illustrated inFIG. 5.

FIG. 5 shows a liquid supply system 100 according to an embodiment ofthe invention in greater detail. The liquid supply system may compriseany sort of liquid containment system LCS such as described above, forexample. The liquid supply system 100 forms part of the lithographicprojection apparatus. The liquid supply system 100 is designed so that astandard water source 80, for example a main supply of water, can beused as the immersion liquid source. However, other liquids may also beused, in which case re-circulation as described below is more likely tobe used and purification may become more important.

Main supply water should require treatment by a liquid purifier beforeit is suitable as an immersion liquid. Other immersion liquids alsorequire such treatment especially if recycled as contamination can occurduring use. In an embodiment, the purifier may be a distillation unit120 and/or a demineralizer 130 and/or a de-hydrocarbonating unit 140 forreducing the hydrocarbon content of the liquid and/or a filter 150. Thedemineralizer 130 may be of any sort such as a reverse osmosis unit, ionexchange unit or electric de-ionization unit, or a combination of two ormore of these units. The demineralizer typically reduces the content ofionic compounds in water or an aqueous solution such that the electricalconductivity of the water or the aqueous solution is between 0.055microSiemens/cm and 0.5 microSiemens/cm. The demineralizer may alsoreduce the silica content to 500 ppt or less, or to 100 ppt or less.

The de-hydrocarbonating unit 140 configured to reduce the hydrocarboncontent of the liquid may be of the type which absorbs the hydrocarbons(e.g. charcoal or polymeric materials) or by combination of a UV lightsource and an ion exchanger. This unit 140 typically reduces the contentof organic compounds in water or an aqueous solution to 5 ppb or less,for example to 3 ppb or less or to 2 ppb or less, to 1 ppb or less or to0.5 ppb or less. The demineralizer 130 will in any case remove some ofthe hydrocarbons.

The filter 150 typically reduces the particle content of the immersionliquid to 2 particle/ml or less, to 1 particle/ml or less, or to 0.5particle/ml or less, wherein a particle is defined as a particle havingat least one dimension of 50 nm or greater. In an embodiment, the filter150 is dynamically isolated from one or more of the other components inthe liquid supply system. Typically, the filter 150 is dynamicallyisolated from components in the liquid supply system, which may causemechanical shock. The filter 150, together with any hosing andcomponents downstream of the filter may, for example, be dynamicallyisolated from any components in the system causing mechanical shocksand/or vibrations, for example motors, switching valves, moving partsand turbulent gas flow.

Before entering a liquid containment system LCS, the liquid passesthrough a gas content reduction device 160. The reduction in the gascontent decreases the likelihood of bubble formation and the gas contentreduction device therefore acts as a bubble reduction device. The gascontent reduction device 160 typically reduces the dissolved oxygencontent of the immersion liquid to 15 ppb or less, to 10 ppb or less orto 5 ppb or less. The gas content reduction device 160 may work usingultra sonic waves as described in U.S. patent application Ser. No.10/860,662, hereby incorporated in its entirety by reference, or onsimilar principles using mega sonic waves (about 1 MHz) which avoid someof the disadvantages of ultra sonic waves (which can lead to cavitationand bubble collision with walls resulting in small particles breakingoff the walls and contaminating the liquid). Other gas content reductiondevices are also possible, for example those described in the abovementioned United States patent application as well as the use ofmembranes perhaps in combination with a vacuum or by purging the liquidwith a low solubility gas, such as helium. Membranes are already usedfor removal of gasses from liquids in fields such as microelectronics,pharmaceutical and power applications. The liquid is pumped through abundle of semiporous membrane tubing. The pores of the membrane aresized so that the liquid cannot pass through them but the gasses to beremoved can. Thus the liquid is degassed. The process can be acceleratedby applying to the outside of the tubing a low pressure. Liqui-Cel™Membrane Contractors available from Membrana-Charlotte, a division ofCelgard Inc. of Charlotte, N.C., USA are, for example, suitable for thispurpose.

Purging with a low solubility gas is a known technique applied in highperformance liquid chromatography (HPLC) to prevent gas bubble trappingin a reciprocating pump head. When the low solubility gas is purgedthrough the liquid, it drives out other gases, such as carbon dioxideand oxygen.

After use in the liquid containment system LCS, the immersion liquid maybe disposed of through a drain 200. Alternatively, the immersion liquid(or part thereof) which has already been used in the liquid containmentsystem LCS may be recycled to pass through the liquid containment systemagain (via conduit 115) either with or without passing through all orsome components of the liquid purifier. The liquid purifier may be madeup of other components and the distillation unit 120, demineralizer 130,de-hydrocarbonating unit 140 and filter 150 may be positioned in anyorder.

Recycling of immersion liquid which has not yet passed through theliquid containment system LCS is also envisaged. For example, liquid maybe extracted from the liquid purifier after having passed through one ormore of the components, and recycled via conduit 115 to enter the liquidpurifier again at a location further up-stream. In this way, theimmersion liquid passes through at least one of the components of theliquid purifier more than once before entering the liquid containmentsystem. This embodiment has an advantage that an improved immersionliquid purity may be achieved.

Recycling the immersion liquid, either before or after passing throughthe liquid containment system, also enables the immersion liquid to bekept flowing at all times, even when there is no flow through outlet200. This helps to avoid the presence of stagnant liquid in the system,which is an advantage since stagnant liquid (such as water) is known tobe prone to contamination due to, for example, leaching fromconstruction materials.

In FIG. 5, liquid pumps used to re-circulate immersion liquid and tocirculate liquid in the liquid containment system LCS are notillustrated.

The liquid supply system 100 of FIG. 5 also has several measuresintended for the reduction or elimination of growth of lifeforms in theimmersion liquid. Even very low levels of such lifeforms in a main watersupply 80 may lead to contamination of the liquid supply system 100.Such lifeforms can include algae, bacteria and fungi.

There are at least three main ways to reduce the growth of suchlifeforms which are illustrated in FIG. 5. It will be appreciated thatthese ways may be used individually or in any combination. The firstway, effective for algae and other green plants, is to ensure thatliquid is not irradiated with visible light, for example by ensuringthat the conduits 110, 115 transporting water in the liquid supplysystem 100 are manufactured of a material which is non-transparent tovisible light. Alternatively, the conduits 110, 115 may be clad in amaterial which does not transmit visible light. Alternatively or inaddition, the entire liquid supply system 100 or even the wholeapparatus may be housed in a container or enclosure (such as a room) 180which is not transparent to visible light. In this way, the organisms inthe liquid cannot photosynthesize and therefore cannot grow or increase.Suitable non-visible light transmissive materials are stainless steels,polymers etc.

FIG. 5 also illustrates the use of an ultra-violet source 145 which isused to illuminate the immersion liquid. The UV source 145 is used toilluminate the liquid before it passes through the liquid containmentsystem LCS such that it is a separate illumination system to theprojection beam PB (which is used to image the substrate W). The UVsource 145 may be positioned anywhere in the liquid supply system 100upstream of the liquid containment system LCS. The UV source killslifeforms which are then removed from the liquid by a particle filter,for example filter 150. A suitable pore size for the filter is 0.03 to2.0 μm though other sizes may also be used, for example 0.1 to 2.0 μm.

A further way of reducing the effect of organisms on the lithographicprojection apparatus is to add a lifeform-growth inhibiting chemicalinto the immersion liquid (which, in the case illustrated in FIG. 5, iswater). This is achieved using a lifeform-growth inhibiting chemicaladding device 147 which may be positioned upstream or downstream of anyof the other components 120, 130, 140, 145, 150, 160 of the liquidsupply system. Typical chemicals are halogen containing compounds(mostly chlorine or bromine based), alcohols, aldehydes, ozone and heavymetals. The dose level of any such chemical should be very low to ensurethat the immersion liquid purity requirements are met. In an embodiment,lifeform-growth inhibiting chemical is not used in order that theimmersion liquid purity requirements are met.

Of course the adding device 147 may also add other chemicals to theimmersion liquid such as surfactants and wetting agents.

While the embodiment in FIG. 5 is illustrated with the immersion liquidfirst being distilled, then de-mineralized then dehydrocarbonated andthen irradiated with UV, before being filtered and finally de-gassed(i.e. de-bubbled), this may happen in any order. Furthermore, chemicalsmay be added to the liquid at any stage upstream of the liquidconfinement system LCS and re-circulated liquid may also be added at anystage upstream of the liquid confinement system LCS. Where there-circulated liquid is added will be dependent upon its purity. In theexample illustrated in FIG. 5, solid lines indicate the re-circulatedliquid is added downstream of the adding device 147, the distillationunit 120, the de-mineralizing unit 130, the de-hydrocarbonating unit140, the UV source 145, the filter 150 and the gas content reductiondevice 160. The dashed lines show alternative positions at whichrecycled liquid may be added. In an implementation, the recycled liquidis added upstream of, at least, the filter 150.

In an embodiment of the invention, the liquid purifier purifies animmersion liquid which is water or an aqueous solution so that theimmersion liquid has one or more of the properties (a) to (f) set outbelow. In an embodiment of the invention, the immersion liquid has oneor more of the following properties (a) to (f):

(a) an electrical conductivity of from 0.055 microSiemens/cm to 0.5microSiemens/cm;

(b) a pH of from 6 to 8;

(c) a content of organic compounds of 1 ppb or less;

(d) a particle content of no more than 0.5 particles having a dimensionof 50 nm or greater per ml of immersion liquid;

(e) a dissolved oxygen concentration of 5 ppb or less; and

(f) a silica content of 100 ppt or less.

The electrical conductivity of the immersion liquid is typicallycontrolled using a demineralizer, for example an ion exchanger or anelectrical deionization unit, such that it is from 0.055 microSiemens/cmto 0.5 microSiemens/cm. In an embodiment, the electrical conductivity is0.3 microSiemens/cm or less, for example 0.1 microSiemens/cm or less.The demineralizer can also be used to control the content of silica inthe immersion liquid. In an embodiment, the silica content is 500 ppt orless, for example 200 ppt or less, 100 ppt or less, 90 ppt or less, oreven 80 ppt or less.

The pH of the immersion liquid may be controlled by any suitable means.Typically, if main supply water purified using a liquid purifier inaccordance with the above described embodiments is used, the pH will bewithin the range of 5 to 8, or of 6 to 8. If additives are included inthe immersion liquid, the amount of such additives should be controlledsuch that the pH of the immersion liquid remains between 5 and 8. Thedesired pH can, alternatively, be achieved by adding a suitable bufferusing, for example, adding device 147. The pH should be controlled bylimiting the presence of components which may alter the pH of the liquid(e.g., water or aqueous solution). This is often preferred to theaddition of, for example, buffers, since the presence of a buffer mayaffect the purity of the immersion liquid in other ways.

The concentration of organic compounds in the immersion liquid istypically controlled by a de-hydrocarbonating unit 140 configured toreduce the hydrocarbon content. Similarly, the number of particlespresent in the immersion liquid can be controlled using filters. Theparticle content of the immersion liquid is the content of particleshaving a size larger than the lowest feature size in the lithographyprocess. Thus, the particle content is the content of particles havingat least one dimension of 50 nm or greater.

The oxygen content of the immersion liquid is typically controlled usinga gas content reduction device as described above. In an embodiment, theoxygen content is reduced to 15 ppb or less, to 10 ppb or less, to 7 ppbor less, to 5 ppb or less, to 4 ppb or less, or to 3 ppb or less.

The liquid supply system 100 may optionally comprise a measuring device(not depicted in FIG. 5 but may be placed anywhere in the liquid supplysystem 100, in an embodiment between the liquid containment system LCSand the gas content reduction device 160) which can be used to measureone or more of the properties (a) to (f) of the immersion liquid. Such ameasuring device may, for example, be located downstream of at leastone, and, in an embodiment, all, of the components 120, 130, 140, 145,150 and 160 of the liquid supply system. An off-line measuring devicemay also be employed, in which a sample of liquid is extracted from asuitable sampling point in the liquid supply system and fed to theoff-line measuring device. The measuring device will, in one embodiment,comprise one or more measuring devices selected from an electricalconductivity measuring device, a pH sensor, a TOC analyzer, a particlecounter, an oxygen sensor and a total silica measuring device. A bubblemeasuring device may also be used. Suitable techniques for measuringeach of the properties (a) to (f) will be familiar to the skilled personin the art.

In an embodiment, there is provided a lithographic projection apparatuscomprising: an illumination system configured to condition a radiationbeam; a support constructed to hold a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; and a liquid supply system configured to at least partly filla space between the projection system and the substrate with a liquid,the liquid supply system comprising a liquid purifier configured topurify the liquid.

In an embodiment, the liquid purifier comprises a distillation unit. Inan embodiment, the liquid purifier comprises a de-hydrocarbonating unitconfigured to reduce the hydrocarbon content of the liquid. In anembodiment, the liquid purifier comprises a demineralizer. In anembodiment, the demineralizer comprises a reverse osmosis unit, an ionexchanger or a de-ionization unit. In an embodiment, the liquid purifiercomprises a filter. In an embodiment, the filter is dynamically isolatedfrom one or more other components in the liquid supply system. In anembodiment, the liquid supply system includes a re-circulation mechanismconfigured to re-use liquid in the space without purifying the liquidfor a re-use. In an embodiment, the liquid supply system includes are-circulation mechanism configured to re-use liquid in the space andthe liquid is partly or fully purified for a re-use. In an embodiment,the liquid supply system further comprises a circulation mechanismconfigured to provide liquid from the liquid purifier to the space. Inan embodiment, the liquid is water or an aqueous solution and the liquidpurifier is configured to purify the water or aqueous solution such thatit has one or more of the following properties (a) to (f): (a) anelectrical conductivity of from 0.055 microSiemens/cm to 0.5microSiemens/cm; (b) a pH of from 5 to 8; (c) a content of organiccompounds of 5 ppb or less; (d) a particle content of no more than 2particles having a dimension of 50 nm or greater per ml of liquid; (e) adissolved oxygen concentration of 15 ppb or less; and (f) a silicacontent of 500 ppt or less. In an embodiment, the pH of the liquid isfrom 6 to 8, the content of organic compounds of the liquid is 1 ppb orless, the particle content of the liquid is no more than 0.5 particleshaving a dimension of 50 nm or greater per ml of liquid, the dissolvedoxygen concentration of the liquid is 5 ppb or less, the silica contentof the liquid is 100 ppt or less, or any combination of the foregoing.In an embodiment, the liquid supply system comprises an ultra-violetsource configured to irradiate the liquid prior to entry into the space.In an embodiment, the liquid supply system comprises a container orenclosure non-transparent to visible light surrounding the liquid supplysystem. In an embodiment, the liquid supply system comprises conduitswhich are non-transparent to visible light configured to supply theliquid from a liquid source to the space. In an embodiment, the liquidsupply system comprises a device configured to add a lifeform-growthinhibiting chemical to the liquid.

In an embodiment, there is provided a lithographic projection apparatuscomprising: an illumination system configured to condition a radiationbeam; a support constructed to hold a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; and a liquid supply system configured to at least partly filla space between the projection system and the substrate with a liquid,the liquid supply system comprising an ultra-violet radiation sourceconfigured to irradiate the liquid prior to entry into the space.

In an embodiment, the ultra-violet radiation is selected to killlifeforms in the liquid. In an embodiment, the liquid supply systemcomprises a particle filter configured to remove lifeforms killed by theultra-violet radiation.

In an embodiment, there is provided a lithographic projection apparatuscomprising: an illumination system configured to condition a radiationbeam; a support constructed to hold a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; and a liquid supply system configured to at least partly filla space between the projection system and the substrate with a liquid,the liquid supply system comprises a component configured to prevent theliquid from being irradiated by visible light.

In an embodiment, the component comprises a container or enclosurenon-transparent to visible light surrounding the liquid supply system.In an embodiment, the component comprises conduits which arenon-transparent to visible light configured to supply the liquid from aliquid source to the space.

In an embodiment, there is provided a lithographic projection apparatuscomprising: an illumination system configured to condition a radiationbeam; a support constructed to hold a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; and a liquid supply system configured to at least partly filla space between the projection system and the substrate with a liquid,the liquid supply system comprising a device configured to add alifeform-growth inhibiting chemical to the liquid.

In an embodiment, the chemical is selected from the group consisting ofhalogen containing compounds, alcohols, aldehydes, ozone and heavymetals. In an embodiment, the device is further configured to add asurfactant, a wetting agent, or both, to the liquid.

In an embodiment, there is provided a liquid for use in a space betweena projection system of an immersion lithographic projection apparatusand a substrate to be imaged, the liquid comprising a lifeform-growthinhibiting chemical.

In an embodiment, the chemical is selected from the group consisting ofhalogen containing compounds, alcohols, aldehydes, ozone and heavymetals. In an embodiment, the liquid further comprises a surfactant, awetting agent, or both.

In an embodiment, there is provided a lithographic projection apparatuscomprising: an illumination system configured to condition a radiationbeam; a support constructed to hold a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; and a liquid supply system configured to at least partly filla space between the projection system and the substrate with a liquid,the liquid comprising a lifeform-growth inhibiting chemical.

In an embodiment, the chemical is selected from the group consisting ofhalogen containing compounds, alcohols, aldehydes, ozone and heavymetals. In an embodiment, the liquid further comprises a surfactant, awetting agent, or both.

In an embodiment, there is provided a lithographic projection apparatuscomprising: an illumination system configured to condition a radiationbeam; a support constructed to hold a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate; and a liquid supply system configured to at least partly filla space between the projection system and the substrate with a liquid,the liquid being water or an aqueous solution having one or more of thefollowing properties (a) to (f): (a) an electrical conductivity of from0.055 microSiemens/cm to 0.5 microSiemens/cm; (b) a pH of from 5 to 8 orfrom 6 to 8; (c) a content of organic compounds of 5 ppb or less or of 1ppb or less; (d) a particle content of no more than 2 particles having adimension of 50 nm or greater per ml of liquid, or of no more than 0.5particles having a dimension of 50 nm or greater per ml of liquid; (e) adissolved oxygen concentration of 15 ppb or less or of 5 ppb or less;and (f) a silica content of 500 ppt or less or of 100 ppt or less.

In an embodiment, the pH of the liquid is from 6 to 8, the content oforganic compounds of the liquid is 1 ppb or less, the particle contentof the liquid is no more than 0.5 particles having a dimension of 50 nmor greater per ml of liquid, the dissolved oxygen concentration of theliquid is 5 ppb or less, the silica content of the liquid is 100 ppt orless, or any combination of the foregoing. In an embodiment, the liquidsupply system comprises a liquid containment system containing theliquid.

In an embodiment, there is provided a liquid for use in a space betweena projection system of an immersion lithographic projection apparatusand a substrate to be imaged, wherein the liquid has one or more of thefollowing properties (a) to (f): (a) an electrical conductivity of from0.055 microSiemens/cm to 0.5 microsiemens/cm; (b) a pH of from 5 to 8 orfrom 6 to 8; (c) a content of organic compounds of 5 ppb or less or of 1ppb or less; (d) a particle content of no more than 2 particles having adimension of 50 nm or greater per ml of liquid, or of no more than 0.5particles having a dimension of 50 nm or greater per ml of liquid; (e) adissolved oxygen concentration of 15 ppb or less or of 5 ppb or less;and (f) a silica content of 500 ppt or less or of 100 ppt or less.

In an embodiment, the pH of the liquid is from 6 to 8, the content oforganic compounds of the liquid is 1 ppb or less, the particle contentof the liquid is no more than 0.5 particles having a dimension of 50 nmor greater per ml of liquid, the dissolved oxygen concentration of theliquid is 5 ppb or less, the silica content of the liquid is 100 ppt orless, or any combination of the foregoing.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

The invention claimed is:
 1. A lithographic projection apparatuscomprising: a substrate table constructed to hold a substrate; aprojection system configured to project a patterned radiation beam ontoa target portion of the substrate; a liquid supply system configured toat least partly fill a space between the projection system and thesubstrate with a liquid, the liquid supply system comprising anultra-violet radiation source configured to irradiate the liquid priorto entry into the space; and a dispenser configured to add a chemical tothe liquid in the liquid supply system.
 2. The apparatus according toclaim 1, further comprising a filter configured to separate particlesfrom the liquid.
 3. The apparatus according to claim 1, furthercomprising a degasser configured to separate gas and liquid prior toentry of the liquid into the space.
 4. The apparatus according to claim3, wherein the degasser comprises a membrane configured to contact theliquid to separate gas and liquid.
 5. The apparatus according to claim1, further comprising a measurement device configured to measure aproperty of the liquid indicative of contamination.
 6. The apparatusaccording to claim 5, wherein the property is an electrical property ofthe liquid.
 7. The apparatus according to claim 5, wherein the liquidsupply system comprises a liquid confinement structure configured to atleast partly confine the liquid within the space and the measurementdevice is configured to measure the property of the liquid in a flowpath between the liquid confinement structure and a gas contentreduction device configured to reduce the gas content of the liquid. 8.The apparatus according to claim 1, wherein the ultra-violet radiationis selected to kill lifeforms in the liquid and further comprising aparticle filter configured to capture lifeforms killed by theultra-violet radiation.
 9. The apparatus according to claim 1, furthercomprising an ion exchange material configured to contact the irradiatedliquid.
 10. The apparatus according to claim 1, wherein the dispenser isconfigured to add ozone, a halogen containing compound, an alcohol, analdehyde, or a heavy metal, to the liquid supply system.
 11. Theapparatus according to claim 1, wherein the dispenser is configured toadd a surfactant, a wetting agent, or both, to the liquid supply system.12. A lithographic projection apparatus comprising: a movable table; aprojection system configured to project a patterned radiation beam ontoa target portion of a substrate; a liquid supply system configured to atleast partly fill a space between the projection system and the tablewith a liquid; a member having: an opening, in a bottom surface of themember, configured to remove at least part of the immersion liquid fromthe space, and an open aperture located underneath a lower surface ofthe projection system and above the table, a cross-sectional dimensionof the aperture being smaller than a cross-sectional dimensional of thelower surface of the projection system, the open aperture configured toallow the liquid to flow therethrouqh between above the open apertureand below the open aperture, and the open aperture configured to allowthe radiation beam to pass therethrouqh; an inlet configured to supplythe immersion liquid to the space at a position located above theaperture; an ultra-violet radiation source configured to irradiate theliquid prior to entry of the liquid into the space; a filter configuredto separate particles from the liquid; and a degasser configured toseparate gas and liquid.
 13. The apparatus according to claim 12,further comprising a measurement device configured to measure a propertyof the liquid indicative of contamination.
 14. The apparatus accordingto claim 13, wherein the property is an electrical property of theliquid.
 15. The apparatus according to claim 12, further comprising anion exchange material configured to contact the irradiated liquid. 16.The apparatus according to claim 12, further comprising an adding deviceconfigured to add a surfactant, a wetting agent, or both, to the liquidsupply system.
 17. A device manufacturing method comprising: at leastpartly filling a space between a projection system of an exposureapparatus and a substrate with a liquid; irradiating the liquid prior toentry into the space with ultra-violet radiation; adding a chemical tothe liquid; and projecting a patterned radiation beam through the liquidin the space onto a target portion of the substrate.
 18. The method ofclaim 17, further comprising measuring an electrical property of theliquid.
 19. The method of claim 18, further comprising degassing theliquid prior to entry into the space.
 20. The method of claim 19,further comprising filtering particles in the liquid.
 21. The method ofclaim 20, further comprising degassing the liquid downstream from anoutlet from the space.
 22. A lithographic projection apparatuscomprising: a substrate table constructed to hold a substrate; aprojection system configured to project a patterned radiation beam ontoa target portion of the substrate; and a liquid supply system configuredto at least partly fill a space between the projection system and thesubstrate with a liquid, the liquid supply system comprising: anultra-violet radiation source configured to irradiate the liquid priorto entry into the space, a conduit to route the liquid to an inlet tothe space, and a further conduit that branches off the conduit beforethe inlet and branches into the conduit upstream of where the furtherconduit branches off the conduit, the further conduit configured toroute at least part of the liquid back upstream to a portion of theliquid supply system through which the at least part of the liquid haspassed.